Hard materials, used in such applications as cutting tools for machining and wear inserts in rock drilling tools, have evolved from high-speed tools steels, to cemented carbides, cermets, ceramics, polycrystalline cubic boron nitride (CBN) and polycrystalline diamond (PCD). This evolution was driven by the need for higher hardness, particularly at high temperatures, to reduce wear at higher cutting speeds to increase productivity. But, the increased hardness has been achieved at the expense of fracture toughness, and the low toughness ceramic tools have been limited to applications with low feed rates and uninterrupted cutting.
Ideally, a tool material should possess a combination of high hardness to resist wear and abrasion, high fracture toughness to resist thermal and mechanical shock, and chemical and thermal stability to resist interaction with the stock. The requirements of high hardness and high fracture toughness are not easy to achieve in a single-phase material via control of the microstructure. A fine grain size, for example, enhances hardness, usually at the expense of fracture toughness. Even in two-phase materials designed for wear and abrasion resistance, such as cemented carbides (WC+Co) and cermets (TiC+Ni), the relative amounts of the carbide phase and the binder phase are optimized to achieve a compromised combination of hardness and fracture toughness.